Neutral Beam Probe Research Articles

Application of a gold neutral beam probe and an end-loss energy component analyzer to space potential measurements in a tandem mirror

A Gold Neutral Beam Probe (GNBP) has been investigated from an active diagnostic point of view, and found to be the best way to measure potentials of the barrier and central cell plasmas in the GAMMA 10 tandem mirror. The thermal barrier potential was determined directly by the simultaneous measurements of the on-axis space potentials at both of these locations. Two-dimensional space potentials were obtained by adjusting the energy of the probing beams so that measurements were made along different chords through the plasma cross-section. A newly developed diagnostic device, a cylindrical End-Loss Energy Component Analyzer (ELECA), was also used to measure the plug potential, and was suitable for determining the distribution functions of end-loss ions. This diagnostic can also be useful as a compact energy analyzer for a beam probe diagnostic.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Edge density profile variation during a magnetic axis shift in the CHS heliotron/torsatron

Edge density profiles were measured during natural and forced magnetic axis shifts in the lowaspect-ratio heliotron/torsatron CHS, using an 8 keV fast neutral lithium beam probe. The Shafranov shift of the magnetic axis (and hence the dislocation of the LCFS) that was negligible for a low density ECH discharge became substantial (∼4.5 cm) for a high density NBI discharge (〈ne〉∼2.5× 1013cm−3) and the corresponding radius of the LCFS increased about 1.2 cm in major radius compared to the vacuum case. For NBI discharges with different settings of the vacuum magnetic axisRax (fixed during the discharge), the measured edge density profiles indicated reasonable agreement between the theoretically and experimentally obtained LCFS radii for 90 cm <Rax<101.6cm, while forRax<90 cm the measured radius was 10% larger than expected. When a change ofRax from 94.7 cm to 89.9 cm during a discharge was imposed externally, a well behaved plasma boundary moved inward smoothly by about 7.5 cm, while the steepness of the edge density profile changed for different values ofRax. The steepest profile was attained forRax=92.1 cm when the highest energy content and average density were achieved.

Laser blow-off lithium beam probing with high temporal resolution for edge plasma diagnostics

Laser blow-off neutral lithium beam probing with high temporal resolution has been developed to measure edge electron density profiles in a plasma. A new electron density reconstruction method, taking account of the transport of a laser blow-off beam in the plasma, improves the accuracy of the density measurement. The use of a camera system with a gate function makes it possible for us to measure the density profile with a high temporal resolution of 10–50 μs. The optimum condition for shutter timing and period to observe the Li i emission profile is discussed from the viewpoints of the beam penetration into the plasma and the accuracy of measurement. A rapid temporal evolution of the edge electron density profile during limiter biasing in a small tokamak HYBTOK-II was successfully obtained by this system.

Edge plasma density reconstruction for fast monoenergetic lithium beam probing

Two different electron density reconstruction methods for 8-keV neutral lithium beam probing have been developed for the Compact Helical System (CHS). Density dependences on emission and ionization processes are included by using effective rate coefficients obtained from the collisional radiative model. Since the two methods differ in the way the local beam density in the plasma is determined, the methods have different applicable electron densities. The beam attenuation is calculated by iteration from the electron density profile in method I. In method II, the beam remainder at the observation point z is determined by integrating the Li i emission intensity from z toward the position of emission tail-off. At the emission tail-off, the fast lithium beam is completely attenuated. Selecting an appropriate method enables us to obtain edge electron density profile well inside the last closed flux surface for various ranges of plasma densities (1012–5×1013 cm−3). The electron density profiles reconstructed by these two different methods are in good agreement with each other and are consistent with results from ruby laser Thomson scattering.

Simultaneous Measurements of Plasma Potential and Density Fluctuations in a Bumpy Torus by a Heavy Neutral Beam Probe

A heavy neutral beam probe was developed for simultaneous measurements of local space potential and density fluctuations in a plasma. It was applied to both electron cyclotron heated (ECH) and radio frequency heated (RF) plasmas in the bumpy torus NBT-1M, and the feasibility of this method was successfully demonstrated by observing a low frequency m=1 interchange mode which was rotating poloidally at a frequency near the E×B drift frequency. It was shown that use of this probe results in extension of the range of fluctuation measurement by beam probe technique to relatively low temperature plasmas. It is applicable to a plasma with a strong RF field with which Langmuir probes cannot be used.

Measurement of the edge plasma density profile in CHS by fast neutral lithium beam probing

A fast (8 keV) neutral lithium beam probe was developed for the measurement of edge density profile in the heliotron/torsatron CHS. It was designed to attain good penetration and to be compatible with good spatial (≈0.01 m) and temporal (≈3 ms) resolution. Two complementary methods of density determination from the beam emission signal were used. For the low density region with (nel)

Reconstruction of Edge-Plasma Density Profiles by Neutral Beam Probe Spectroscopy

Thermal neutral beam probing combined with a spectroscopic technique is numerically developed to reconstruct electron-density profiles of edge plasmas. Although the reconstruction has been performed by assuming all beam atoms to have a thermal velocity, the influence of a beam-velocity distribution has not received as much attention. It is shown that the method which neglects the beam-velocity distribution reconstructs electron densities lower than the true values by a factor of ∼2 in the density range up to ∼1×1013 cm-3, while the procedure which takes into account the velocity distribution yields true density profiles in the same density range. Some results of the beam probe mounted in the Heliotron-E device are also described, and good agreement with Langmuir probe measurements is found.

Numerical study of neutral beam probe spectroscopy for edge fluctuation measurements

A thermal neutral beam probing combined with a spectroscopic technique is numerically demonstrated to be available to the measurement of local electron-density fluctuations in edge plasmas, although the spatial attenuation of the beam due to the electron-impact ionization is not negligible. It is found that the fluctuations are accurately measured in the region where the density is lower than (4–6) × 1012 cm−3, and that this critical density decreases slightly for the fluctuations which propagate in the beam direction with phase velocities comparable to a thermal speed of the beam. Some results of the beam probe mounted on the TEXTOR tokamak are also described and good agreement is found between numerical and experimental results.

Enhancement of the thermal barrier depth and two-dimensional potential profiles in GAMMA 10

Gold neutral beam probes are used for the determination of the space potentials at the thermal barrier cells and the central cell of the GAMMA 10 tandem mirror. The thermal barrier potential is determined by simultaneous measurements of the on-axis space potentials at both locations. Equipotential contours over a cross-section at the barrier are measured by sweeping the probing beams with different beam energies. The enhancement of the thermal barrier depth and of the neoclassical radial ion transport in relation with two-dimensional space potential profiles has been studied in GAMMA 10 for the case of low density experiments.

レーザー分光によるプラズマ計測

Recent development of plasma diagnostics by laser spectroscopy are reviewed. In paticular, some current studies by use of dye-laser are presented; the method of laser-induced fluorescence (LIF) and beam-probe spectroscopy. A new diagnostic method using both techniques of neutral beam probing and LIF has been developed to measure the spatial electron density distribution. A novel technique to measure the magnetic field is also demonstrated, which is determined from the Zeeman splitting of the LiI resonance line with good time and spatial resolutions.

Thermal barrier potential and two-dimensional space-potential measurements with gold neutral-beam probes in GAMMA 10

Gold neutral-beam probes are used for the space-potential measurements in the barrier and in the central cells of the GAMMA 10 tandem mirror. The thermal barrier potential depth is directly observed by simultaneous measurements of the on-axis space potentials at both locations. Two-dimensional space potentials in the barrier region are measured by sweeping the probing beams with different beam energies.

The PBX-M 80 kV neutral probe beam

The PBX-M Neutral Probe Beam Project was initiated to design, fabricate, acceptance-test, install, and commission an 80 kV, 2.7 A neutral probe beam for use in PBX-M q-profile measurements. The 100 ms, H° output beam is modulated at 1 kHz and has a pulse repetition rate of 3 min. The beam has a divergence of 0.5° and a focal length of 400 cm. The injection angle can be varied about 25°, from near perpendicular to outboard tangential. The beamline is shielded from magnetic fields and interfaces to the existing mechanical, electrical, and control systems of PBX-M.

Edge fluctuation measurements in textor with a thermal neutral beam probe

Simultaneous measurements of radial profiles of the electron density and its fluctuations in the TEXTOR scrape-off layer have been performed with a thermal neutral lithiam beam probe. Fluctuation levels and frequency spectra are compared with each other for different plasma states, e.g. helium and hydrogen plasmas, detatched and attached plasmas. The influence of the pump limiter is also investigated. In all plasma states, broadband fluctuations with a similar frequency spectrum are observed. The fluctuation levels are well correlated with density gradient scale lengths and with line-averaged electron densities. The different features between the various are also presented.

Autoneutralizing anions as a neutral beam probe for the analysis of insulating materials

A well-focused beam of fast (2.5–25 keV) sulfur hexafluoride neutral molecules has been developed based upon the autoneutralizing sulfur hexafluoride gas-phase anion. This molecular beam has been used to sputter gas-phase ions from dry solids with excellent results from a wide range of materials that include plastics and pharmaceuticals. Most pharmaceuticals have yielded predominantly protonated parent ions, with fragments typical of the molecular structure. Detection limits for these pharmaceuticals are around 1 ng.

Neutral probe beam q-profile measurements in PDX and PBX-M

Using the fast ion diagnostic experiment technique, a neutral probe beam (NPB) can be aimed to inject tangent to a magnetic surface. The resultant ion orbit shifts, due to conservation of canonical toroidal angular momentum, can be measured with a multi-sight-line charge exchange analyzer to yield direct measurements of radial magnetic flux profiles, current-density profiles, the radial position of the magnetic axis, flux surface inner and outer edges, q profiles, and central-q time dependencies. An extensive error analysis was performed on previous PDX q measurements in circular plasmas and the resulting estimated contributions of various systematic effects are discussed. Preliminary results of fast ion orbit shift measurements at early times in indented PBX-M plasmas are given. Methods for increasing the absolute experimental precision of similar measurements in progress on PBX-M are discussed.

Neutral-beam diagnostic for fast confined alpha particles in a burning plasma: Application on CIT

Measurement of the slowing-down distribution for fast confined alpha particles is essential for a D–T burning-plasma experiment. Quantitative estimates of signal level for a fast neutral-beam probe using a two-electron-capture method of measuring the alpha-particle velocity distribution are presented as applied to the proposed tokamak CIT (Compact Ignition Tokamak). The best probe is found to be a fast ground-state helium-atom beam because of its relatively good penetration into a dense plasma and because of the large cross section for two-electron capture; fast He0 can be produced in useful quantities from fast HeH+, which is sufficient to allow time-resolved measurements of the alpha-particle velocity distribution in CIT.

Structure of the scrape-off layer and particle fluxes in a limiter tokamak

The determination of electron density profiles across the scrape-off layer (SOL) with “neutral beam probes” (Li) in combination with a variable poloidal limiter system shows that the edge densities are not axisymmetric. The profile near the limiter inside the SOL is much steeper than that near the symmetry plane of the flux tube. This result is reproduced by a 2-D MHD model of the SOL where particle sources coming from neutrals, which are released from the limiter, are essential. By using the profile measurements, a particle flux balance can be calculated for radial diffusion, toroidal flow and recycling of neutrals which confirms the picture.

Ion rotational velocity of a field-reversed configuration plasma measured by neutral beam probe spectroscopy

The ion rotational angular velocity Ω and the ion temperature Ti of a translated field-reversed configuration (FRC) plasma are measured using neutral beam probe spectroscopy. The value of Ω is ∼(1.0∼1.2)×Ω* at the onset time of the n=2 rotational instability, where Ω* is the ion diamagnetic frequency for a rigid-rotor equilibrium. The ion rotational direction is the same as the ion diamagnetic direction. The value of Ω is smaller than the angular frequency ωre of the n=2 instability, which can yield experimental evidence of the ion kinetic effects on the n=2 instability in the FRC plasma. When the octupole field is applied to the plasma in order to suppress the n=2 deformation, Ω is slightly reduced. The ion temperature Ti is ∼70 eV at the onset time of the n=2 instability.

Neutral beam probe spectroscopy for the measurement of local plasma density fluctuations

A neutral Li beam probing combined with a spectroscopic technique to measure local electron-density fluctuations is experimentally demonstrated in an ECR test plasma. The density fluctuations of up to 180 kHz are excited and controlled by applying periodic bursts of sinusoidal waves to a grid. The electron-density fluctuation levels thus obtained are found to agree well with those measured with a Langmuir probe. It is also suggested that this method can be applied to plasmas whose densities are in a wide range of 1010–1013 cm−3.

Measurements of ion temperature and drift energy of hydrogen plasma by neutral beam probe spectroscopy

The ion temperature and the energy of gross drift motion of a fully ionized hydrogen plasma with a field-reversed configuration are measured from the observed Doppler profile of the Hβ line from charge exchange neutrals which are generated by a pulsed intense hydrogen beam with energy ∼400 eV. At the beam energy, the generated neutrals are almost hydrogen atoms in the ground state and they emit spectral lines via electron impact excitation. A Monte Carlo analysis shows that the obtained Doppler temperature of the charge exchange neutrals agree with the ion temperature within 6% accuracy. The measured ion temperature and drift energy of the plasma are typically 100 and 10 eV, respectively.